904L alloy is a super austenitic stainless steel designed for a variety of medium and high corrosion resistance environments. The high content of chromium and nickel, coupled with molybdenum and copper as a combination of components, ensure excellent corrosion resistance of the product.

With a high alloy chemical composition of 25% nickel and 4.5% molybdenum, 904L is superior to 316L and 317L stainless steels in resistance to chloride stress corrosion cracking, pitting and corrosion resistance.

The 904L alloy was originally developed for corrosion resistance in dilute sulfuric acid. It also has good corrosion resistance to other inorganic acids (such as hot phosphoric acid) and most organic acids.

The 904L alloy is easily welded and machined in standard shop manufacturing practices

Grade 904L stainless steels have excellent resistance to warm seawater and chloride attack. The high resistance of grade 904L against stress corrosion cracking is due to the presence of high amounts of nickel in its composition. Moreover, the addition of copper to these grades develops resistance to sulphuric acid and other reducing agents in both aggressive and mild conditions.

The corrosion resistance of grade 904L is intermediate between super austenitic grades, with 6% molybdenum content, and standard 316L austenitic grades. Grade 904L is less resistant to nitric acid than grades 304L and 310L, which are free of molybdenum. This steel grade needs to be solution treated following cold working, to achieve maximum stress corrosion cracking resistance under critical environments.

Heat Resistance

Grade 904L stainless steels offer good oxidation resistance. However, the structural stability of this grade collapses at high temperatures, particularly above 400°C.

Heat Treatment

Grade 904L stainless steels can be solution heat-treated at 1090 to 1175°C, following by rapid cooling. Thermal treatment is suitable for hardening these grades.

Welding

Welding of grade 904L stainless steels can be performed using all conventional methods. This grade does not require pre-heat and post-weld heat treatments. Grade 904L can be subjected to hot cracking in constrained weldment. Grade 904L electrodes and rods are used for welding grade 904L steels according to AS 1554.6.

Fabrication

Grade 904L stainless steels are high purity steels with low sulphur content. They can be machined using any standard methods. These grades can be readily bent to a small radius under cold conditions. Although subsequent annealing is not required in most cases, it should be carried out when the fabrication is performed under severe stress corrosion cracking conditions.

• Wiring in electrostatic precipitators
• Oil refinery components
• Seawater cooling devices
• Gas scrubbing plants
• Pulp and paper processing industries
• Acetic, phosphoric and sulphuric acid processing plant